Recently, in the field of production of optical parts, such as optical elements that radiate or pass ultraviolet rays, there has been a tendency toward a significant increase in brightness and intensity of light. However, epoxy-type organic materials that are conventionally used for such optical elements have low resistance to ultraviolet rays and can easily change their color to brown after long exposure to ultraviolet radiation. On the other hand, cured bodies of curable silicone compositions that contain polydimethylsiloxanes show high resistance to heat and ultraviolet radiation, and therefore the optical parts that are made from epoxy-type organic resins and are associated with radiation or passage of ultraviolet rays are now being replaced by optical parts made from curable silicone compositions of a polydimethylsiloxane type.
However, a problem associated with the use of curable silicone compositions and cured bodies of curable silicone compositions of a polydimethylsiloxane type is that their refractive index of 1.41 is lower than that of an epoxy resin, and therefore optical parts that utilize cured bodies of the aforementioned compositions cannot provide high brightness of light. It has been found that refractive index can be increased to the level of epoxy-resin type organic materials by introducing compositions that contain phenyl groups, such as polydimethyl-diphenyl siloxane type or polymethylphenyl siloxane type curable silicone compositions.
Even though the aforementioned phenyl-containing curable silicone compositions appeared to be superior to the epoxy-type organic resin materials with regard to ultraviolet (UV)-ray resistance, they still could not reach in their UV radiation resistance the level of the polydimethylsiloxane-type curable silicone compositions. Examples of such copolymers are given in Japanese Unexamined Patent Application Publication (hereinafter Kokai) H10-36511 that discloses copolymers composed of siloxane units that contain cyclohexyl or cyclopentyl groups, as well as silanol and phenyl groups. The publication also shows examples of cured bodies of the aforementioned copolymers. However, since phenyl groups are indispensable constituents of such copolymers, the cured bodies thereof have a high index of refraction, therefore, low resistance to UV rays.
For addition-reaction curing, known curable silicone compositions are compounded with platinum catalysts (see, e.g., Kokai 2000-17176), but when such compositions are exposed to UV radiation, under the effect of UV rays and heat the platinum catalysts contained in the composition are colored, whereby a cured body of the composition is turned into something between brown and yellow.